1. Field of the Invention
The present invention relates to photovoltaic cells and, in particular, to an apparatus and a method for forming a monolithic interconnected photovoltaic module.
2. Description of the Related Art
As is well known in the thin-film semiconductor art, photovoltaic cells that convert solar radiation into usable electrical energy can be fabricated by sandwiching certain semiconductor structures, such as, for example, the amorphous silicon PIN structure disclosed in U.S. Pat. No. 4,064,521, between two electrodes. One of the electrodes typically is transparent to permit solar radiation to reach the semiconductor material. This "front" electrode (or contact) can be comprised of a thin film (i.e., less than 10 micrometers in thickness) of transparent conductive oxide material, such as tin oxide, and usually is formed between a transparent supporting substrate made of glass or plastic and the photovoltaic semiconductor material. The "back" electrode (or contact), which is formed on the surface of the semiconductor material opposite the front electrode, generally comprises a thin film of metal such as, for example, aluminum. Alternatively, the back electrode can be made of a transparent material such as tin oxide.
The voltage produced across the electrodes of a single photovoltaic cell, however, is insufficient for most applications. To achieve a useful power level from photovoltaic semiconductor devices, individual photovoltaic cells must be electrically connected in series in an array referred to herein as a photovoltaic "module." A typical arrangement of series-connected photovoltaic cells is shown in FIG. 1.
FIG. 1 shows photovoltaic module 10 comprised of a plurality of series-connected photovoltaic cells 12 formed on a transparent substrate 14 and subjected to solar radiation 16 passing through substrate 14. Each photovoltaic cell 12 includes a front electrode 18 of transparent conductive oxide, a photovoltaic element 20 made of a semiconductor material, such as, for example, hydrogenated amorphous silicon, and a back electrode 22 of a metal such as aluminum or of a transparent material such as tin oxide. Photovoltaic element 20 can comprise, for example, a PIN structure. Adjacent front electrodes 18 are separated by first grooves 24, which are filled with the semiconductor material of photovoltaic elements 20. The dielectric semiconductor material in first grooves 24 electrically insulates adjacent front electrodes 18. Adjacent photovoltaic elements 20 are separated by second grooves 26, which are filled with the metal of back electrodes 22 to provide a series connection between the front electrode of one cell and the back electrode of an adjacent cell. Adjacent back electrodes 22 are electrically isolated from one another by third grooves 28.
The thin-film photovoltaic module of FIG. 1 typically is manufactured by a deposition and patterning method. One example of a suitable technique for depositing a semiconductor material on a substrate is glow discharge in silane, as described, for example, in U.S. Pat. No. 4,064,521. Several patterning techniques are conventionally known for forming the grooves separating adjacent photovoltaic cells, including silkscreening with resist masks, etching with positive or negative photoresists, mechanical scribing, electrical discharge scribing, and laser scribing. Laser scribing and silkscreening methods have emerged as practical, cost-effective, high-volume processes for manufacturing thin-film semiconductor devices, including amorphous silicon photovoltaic modules. Laser scribing has an additional advantage over silkscreening because it can separate adjacent cells in a multi-cell device by forming separation grooves having a width less than 25 micrometers, compared to the typical silkscreened groove width of approximately 300-500 micrometers. A photovaltaic module fabricated with laser scribing thus has a large percentage of its surface area actively engaged in producing electricity and, consequently, has a higher efficiency than a module fabricated by silkscreening. A method of laser scribing the layers of a photovoltaic module is disclosed in U.S. Pat. No. 4,292,092.
Referring to FIG. 1, a method of fabricating a multi-cell photovoltaic module using laser scribing comprises; depositing a continuous film of transparent conductive oxide on a transparent substrate 14, scribing first grooves 24 to separate the transparent conductive oxide film into front electrodes 18, fabricating a continuous film of semiconductor material on top of front electrodes 18 and in first grooves 24, scribing second grooves 26 parallel and adjacent to first grooves 24 to separate the semiconductor material into individual photovoltaic elements 20 (or "segments") and expose portions of front electrodes 18 at the bottoms of the second grooves, forming a continuous film of metal on segments 20 and in second grooves 26 so that the metal forms electrical connections with front electrodes 18, and then scribing third grooves 28 parallel and adjacent to second grooves 26 to separate and electrically isolate adjacent back electrodes 22.
A distinct disadvantage of photovoltaic modules of the prior art has heretofore been the unavailability of large photovoltaic modules having the flexibility of producing any desired voltage output. By "large photovoltaic modules" it is meant such modules on the order of one foot square and large. By "small voltage output" it is meant 12-15 V or less.
It is well known that the voltage output of a photovoltaic module is directly related to the number of photovoltaic cells connected in series. That is, as the number of cells increases so does the voltage. In a large photovoltaic module, one approach to control the voltage output is to reduce the number of individual cells in the module while increasing their size, specifically their width. Such a module is disclosed in U.S. Pat. No. 4,542,255 issued to Tanner, et al. A disadvantage to the approach disclosed by Tanner, et al. is, however, that the fill factor of the individual cells decreases as the width of the cell increases. Thus, the efficiency, hence power output of the cell decreases as the size of the cells increases.
The present invention is intended to provide a photovoltaic module, and a method for making same, that is a large photovoltaic module, producing an optimal width of the individual cells to maximize their power output.
It is also desirable in some applications, such as the integration of photovoltaic modules into automobile sunroofs, to increase the transmittance of light through the photovoltaic module. This can be accomplished to a certain extent by making both the front and rear contacts (or "electrodes") transparent. Such transparent contacts can be made of, for example, tin oxide. The transmittance of the photovoltaic module can also be increased by reducing the width of each cell in the module to therefore increase the number of scribe lines in the photovoltaic module, hence its light transmittance. As discussed above, decreasing cell width has the effect of increasing the overall voltage of the resulting series-connected photovoltaic module. Such high voltage is not desired for some applications.
It is further intended that the present invention provide a photovoltaic module having individual cells of reduced width while maintaining the output voltage of the photovoltaic module at any desired level.
The present invention also relates to a method for depositing an elongated narrow band conductive pattern. In prior art methods, such as Tanner, et al., such conductive patterns were deposited by a silkscreening method. Silkscreening, however, is wasteful of conductive fluid used to make a conductive pattern, is not easily automated, and produces poor reproducibility. Further, it is difficult to control the thickness of patterns produced.
The present invention is intended to provide a method for depositing an elongated conductive pattern that is not wasteful of conductive fluid used, is susceptible to computer control to produce reproducible patterns and which produces a pattern that has a thickness that can be easily controlled.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims.